Methods are known in the art for producing silicone rubber powder. One such method involves pulverizing cured silicone rubber into a powder after it has been frozen with dry ice. See Japanese Kokai JP 60-81, 227; Soviet Union Patent No. 952895 and Silicone Rubber in Powder Form, by Swanson, Leicht and Wegener, American Chemical Society, Rubber Division, October 1974. Another method is pulverizing cured silicone rubber into a powder using a grinder at ambient temperature. Using these methods, it is difficult to obtain high productivity and uniform micron sized particles. It is also too costly for many applications.
There have also been direct methods developed for producing silicone rubber particles. European Patent Application Publication No. 025510, Method for Producing Silicone Rubber Powder, involves blending a curable liquid silicone rubber composition at temperatures from -60.degree. C. to plus 5.degree. C., maintaining the mixture at this temperature, and then spraying the mixture into hot air, at a temperature of from 80.degree. C. to 200.degree. C. This method uses a curable liquid silicone rubber composition containing a liquid, reactive-group-containing organopolysiloxane with at least two alkenyl groups per polymer, an organopolysiloxane crosslinker containing at least two hydride functionalities per polymer chain, and a catalyst. The technique is designed to produce spherical silicone rubber particles with diameters from several hundred microns to several hundred millimicrons.
Japanese Patent Application JP62-257939, discloses making silicone rubber particles having particle sizes below 20 microns. These particles are made by a process similar to that disclosed in U.S. Pat. No. 4,742,142 using spray drying to isolate the cured silicone rubber particles.
U.S. Pat. No. 4,743,670 is also directed to a method for producing silicone rubber powder. This method involves preparing a heat-curable, liquid silicone rubber dispersion in water at a temperature of from 0.degree.C. to 25.degree. C. in the absence of any surfactant.
U.S. Pat. No. 4,742,142 describes a method for making small silicone rubber powders of uniform shape without grinding or cryofracturing. This method uses a curable liquid silicone rubber composition containing a liquid, reactive-group-containing organopolysiloxane with at least two alkenyl groups per polymer, an organopolysiloxane crosslinker containing at least two hydride functionalities per polymer chain, and a catalyst. The ingredients are blended together at a temperature of from -60.degree. C. to plus 5.degree. C., preferably from -30.degree. C. to 0.degree. C., to prevent the mixture from curing prematurely. The mixture is then emulsified by blending with water and a surfactant at a temperature from 0.degree. C. to 25.degree. C. The emulsion is then dispersed into water and cured at a temperature of at least 25.degree. C. Illustrative examples demonstrate the curing step, using water at 40.degree. C. and 80.degree. C., followed by isolating the particles to produce spherical crosslinked rubber particles of about 10 and 20 microns. This patent by itself and in conjunction with the earlier cited patents teaches that a lower temperature is preferred prior to dispersion.
Large scale commercial production of silicone rubber particles which requires maintaining a dispersion of a curable liquid organopolysiloxane at temperature below 25.degree. C. to prevent premature curing, are a significant disadvantage. It would be more advantageous to be able to prepare cured silicone polymer particles without the need for such low temperature processing.
U.S. Pat. No 4,962,165 describes an ambient temperature process for producing micron sized particles by a somewhat tedious process beginning with small molecular weight materials (monomers) up to oligomers.